1. Field of the Invention
The present invention generally relates to automatic antennas, and particularly to an automatic antenna for a handheld radiotelephone.
2. Prior Art
In recent times, communication terminals have been in great demand because of the widespread use of devices such as cellular phones and pagers for mobile communication. In wireless communication systems, wireless communication terminals transmit signals to base stations. The base stations relay the signals to intended recipient communication terminals all the time, regardless of whether the intended recipient communication terminals are in communication-waiting mode. Portable electronic devices such as mobile phones and cordless telephones generally transmit and receive signals by antenna, and antennas that are used in these devices are in great demand.
Automatic antennas for portable electronic devices can be divided into two categories: built-in antennas and built-out antennas. Built-out antennas are generally exposed outside of the portable electronic device. Built-in antennas are retained in a housing of the portable electronic device, and generally are not exposed out of the portable electronic device. With a built-in antenna, the overall length of the portable electronic device is reduced, and the radiation of magnetic fields is greatly decreased. However, the built-in antenna is subject to shielding phenomena, which can greatly interfere with the transmission and reception of signals. In particular, when the housing of the portable electronic device is made of an alloy such as aluminum combined with magnesium, the shielding phenomena are serious and problematic. Built-out antennas provide better transmission and reception of signals than built-in antennas. However, in most present-day portable electronic devices, the built-out antenna is located adjacent the earphone. There is some scientific research suggesting that frequent use of this kind of portable electronic device exposes the user's brain to much electromagnetic radiation which may be harmful.
A typical built-out antenna can be adjusted between an extended position and a retracted position. The built-out antenna is generally operated by hand. P.R. China Patent Application No. 99110465.X describes a built-out antenna for a mobile phone including a protrusion disposed on one end of the antenna. The protrusion extends out of a housing of the mobile phone. When needed, the antenna can be drawn out of the housing or pressed into the housing by operating the protrusion. However, both of these operations require the user to use both his/her hands together. If the user has only one hand free, he/she cannot operate the antenna. In addition, the protrusion may be unduly thick, which may detract from the aesthetic appearance of the mobile phone.
To solve the above-mentioned problem, some manufacturers utilize electric power to drive the antenna. A mobile phone with this kind of antenna is disclosed in P.R. China Patent Application No. 99802898.3. The antenna has a driving mechanism connected with the electric power source of the mobile phone. The antenna is extended out of the housing by the driving mechanism, and retracted back into the housing by the driving mechanism. However, it is generally necessary for the mobile phone to have an enhanced electric power source. In addition, the driving mechanism is complicated and bulky, and adds to an overall size of the mobile phone. Furthermore, the power consumption of the mobile phone is increased, which adds to the cost of owning the mobile phone.
Therefore, a new antenna device is desired in order to overcome the above-described problems.